1. Field of the Invention
This invention relates to a method for the preparation of a cathode active material, capable of reversibly doping/undoping lithium, and to a method for the preparation of a non-aqueous electrolyte cell employing this cathode active material.
2. Description of Related Art
Nowadays, in keeping up with the recent marked progress in the electronic equipment, researches into re-chargeable secondary cells, as power sources usable conveniently and economically for prolonged time, are underway. Representative of the secondary cells are lead accumulators, alkali accumulators and non-aqueous electrolyte secondary cells.
Of the above secondary cells, lithium ion secondary cells, as non-aqueous electrolyte secondary cells, have such merits as high output and high energy density. The lithium ion secondary cells are made up of a cathode and an anode, including active materials capable of reversibly doping/undoping lithium ions, and a non-aqueous electrolyte.
As the anode active material, metal lithium, lithium alloys, such as Li—Al alloys, electrically conductive high molecular materials, such as polyacetylene or polypyrrole, doped with lithium, inter-layer compounds, having lithium ions captured into crystal lattices, or carbon materials, are routinely used. As the electrolytic solutions, the solutions obtained on dissolving lithium salts in non-protonic organic solvents, are used.
As the cathode active materials, metal oxides or sulfides, or polymers, such as TiS2, MoS2, NbSe2 or V2O5, are used. The discharging reaction of the non-aqueous electrolyte secondary cells, employing these materials, proceeds as lithium ions are eluated into the electrolytic solution in the anode, whilst lithium ions are intercalated into the space between the layers of the cathode active material. In charging, a reaction which is the reverse of the above-described reaction proceeds, such that lithium is intercalated in the cathode. That is, the process of charging/discharging occurs repeatedly by the repetition of the reaction in which lithium ions from the anode make an entrance into and exit from the cathode active material.
As the cathode active materials for the lithium ion secondary cells, LiCoO2, LiNiO2 and LiMn2O4, for example, having a high energy density and a high voltage, are currently used. However, these cathode active materials containing metallic elements having low Clarke number in the composition thereof, are expensive, while suffering from supply difficulties. Moreover, these cathode active materials are relatively high in toxicity and detrimental to environment. For this reason, novel cathode active materials, usable in place of these materials, are searched.
On the other hand, it is proposed to use LiFePO4, having an olivinic structure, as a cathode active material for the lithium ion secondary cells. LiFePO4 has a high volumetric density of 3.6 g/cm3 and is able to develop a high potential of 3.4 V, with the theoretical capacity being as high as 170 mAh/g. In addition, LiFePO4 in an initial state has an electro-chemically undopable Li at a rate of one Li atom per each Fe atom, and hence is a promising material as a cathode active material for the lithium ion secondary cell. Moreover, since LiFePO4 includes iron, as an inexpensive material rich in supply as natural resources, it is lower in cost than LiCoO2, LiNiO2 or LiMn2O4, mentioned above, while being more amenable to environment because of lower toxicity.
However, LiFePO4 is low in electronic conduction rate, such that, if this material is used as a cathode active material, the internal resistance in the cell tends to be increased. The result is that the polarization potential on cell circuit closure is increased due to increased internal resistance of the cell to decrease the cell capacity. Moreover, since the true density of LiFePO4 is lower than that of the conventional cathode material, the charging ratio of the active material cannot be increased sufficiently if LiFePO4 is used as the cathode active material, such that the energy density of the cell cannot be increased sufficiently.
So, a proposal has been made to use a composite material of a carbon material and a compound of an olivinic structure having the general formula of LixFePO4 where 0<x≦1, referred to below as LiFePO4 carbon composite material, as a cathode active material.
Meanwhile, as a method for the preparation of the LiFePO4 carbon composite material, having the olivinic structure, such a method has been proposed which consists in mixing lithium phosphate (Li3PO4) and iron phosphate I (Fe3(PO4)2 or hydrates thereof ((Fe3(PO4)2.nH2O), where n denotes the number of hydrates, adding carbon to the resulting mixture and in sintering the resulting mass at a pre-set temperature, such as 600° C. or thereabouts.
However, Fe in LiFePO4 is in the bivalent state and is liable to oxidation, so that sintering is carried out in an atmosphere containing an inert gas, such as nitrogen. From the operating efficiency, the sintered product is to be taken out from the firing furnace as promptly as possible. For example, in a batch type sintering furnace, the cooling time directly influences the operating ratio of the sintering furnace, whereas, in a belt conveyor type sintering furnace, the cooling time influences the area of the furnace mounting site.
However, if the sintered product is exposed to atmosphere without being cooled sufficiently at the taking-out time, LiFePO4 undergoes the oxidizing reaction shown by the following chemical formula (1):6LiFePO4+3/2O2→2Li3Fe2(PO4)3+Fe2O3  (1)by reaction with atmospheric oxygen to produce impurities to deteriorate the properties of the cathode active material or to interfere with single-phase synthesis of the LiFePO4 carbon composite material. Stated differently, such a temperature management condition for a sintered material which might compromise the operating efficiency and reliable single-phase synthesis has as yet not been established to date.